The present invention relates to coated steel sheets intended for use mainly as steel sheets as automobile bodies, and more particularly to coated steel sheets having superior perforative corrosion resistance, paint adhesion, electrodeposition paintability, press formability, and weldability.
Steel sheets with zinc-based plating are widely used as automobile bodies in order to prevent the strength of automobile bodies from decreasing due to use in a corrosive environment for a long time, and mainly zinc-alloy plated steel sheets such as zinc-nickel-alloy plated steel sheets and zinc-iron-alloy plated steel sheets are used in Japan.
Although zinc-based alloy plating can impart high corrosion resistance to steel sheets by alloying nickel or iron with zinc, it has several problems attributable to being alloy plating.
For example, zinc-nickel alloy plated steel sheets are manufactured by electrogalvanizing and the cost is high because nickel is expensive. Furthermore, the nickel content should be normally adjusted in an extremely narrow range (for example, 12xc2x11 mass %), which makes its manufacture difficult.
On the other hand, zinc-iron-alloy steel sheets can be produced by either electrogalvanizing or hot-dip galvanizing.
However, when zinc-iron-alloy plated steel sheets are manufactured by electrogalvanizing, it entails difficulty in alloy control, i.e. adjusting the iron content in the zinc plating layer in an extremely narrow range, similar to zinc-nickel alloy plated steel sheets. In addition, Fe2+ ions in the plating solution are prone to be oxidized, which destabilizes plating and makes the manufacture difficult. As a result, the cost is high.
Generally, zinc-iron-alloy plated steel sheets are often manufactured by hot-dip galvanizing. When zinc-iron-alloy plated steel sheets are manufactured by hot-dip galvanizing, molten zinc is allowed to adhere to the surface of steel sheets, which are kept at a high temperature to alloy the steel sheets and zinc. However, the quality of plating produced by this method is susceptible to the Al concentration in molten zinc plating baths and to the temperature and duration of the alloying process, and an advanced technology is necessary to manufacture uniform alloy plating layers. As a result, the cost is high also in this case.
As mentioned above, zinc-based-alloy plating has problems that any form of them is difficult to manufacture and the cost is high.
On the other hand, galvanized steel sheets, which have only zinc plating, are less expensive and can be manufactured by either electrogalvanizing or hot-dip galvanizing. However, they are rarely used as automobile bodies. The reason is that zinc plating alone cannot provide enough corrosion resistance, especially when galvanized steel sheets are exposed to a corrosive environment for a long time, the steel sheets tend to develop perforation by corrosion, which causes a problem in terms of guarantee of the strength of automobile bodies. Furthermore, galvanized steel sheets have problems that a quantity of zinc tends to deposit on electrodes in spot welding, which shortens the life of electrodes, and the plates have poor press formability.
Generally, in manufacturing automobile bodies, steel sheets or plated steel sheets are subjected to press forming and welding, and are subjected to a chemical conversion treatment, electrodeposition painting, and spray painting in succession before being used as automobile bodies. It is said the lower parts of doors are most likely to develop perforation by corrosion. The reasons are that steel sheets are folded at the lower parts of doors, and water entering through slits of windows etc. pools in the folded parts, which accelerates the corrosion compared to the other parts of automobile bodies.
Among the treatments applied after the press forming of bodies, the chemical conversion treatment and the electroplating can be applied to the inner surface of doors, but in the spray painting, which is conducted afterwards, the paint cannot reach the inner surface. Therefore, the anti-corrosive effect of the spray painting cannot be expected to be obtained, and corrosion resistance after the electrodeposition painting is important. Among the parts in the door, the folded part (a hem structure) at the lower end of the door that is exposed to the harshest environment does not receive electrodeposition painting and is exposed to the corrosive environment without the protection of electrodeposition painting. Therefore, perforative corrosion resistance is important in both cases where no electrodeposition painting is conducted (no coating) and only electrodeposition painting is conducted (after electrodeposition painting).
With this situation as a background, a technology in which a coating containing magnesium is formed on zinc plating is disclosed as a method of improving corrosion resistance of galvanized steel sheets. For example, Japanese Unexamined Patent Application Publication No. 1-312081 discloses coated metal materials wherein a phosphate coating containing 0.1 mass % or more of magnesium is formed on an electrogalvanized layer.
However, although the coated metal materials which have a phosphate coating containing only magnesium as described in the above-mentioned publication are resistant to rust formation in a salt spray corrosion test, they have insufficient perforative corrosion resistance in a composite cycle corrosion resistance test, which yields similar results to the actual corrosion in automobile bodies.
Furthermore, as surface-treated steel sheets with improved corrosion resistance, there may be mentioned an organic composite coated steel sheet comprising a chromate layer and an organic coating layer containing silica are formed on a zinc-nickel alloy electroplated steel sheet with a coating weight of 20 to 30 g/m2, a zinc hot-dip galvanized steel sheet with a heavy coating weight of 60 g/m2, and an electrogalvanized steel sheet with a heavy coating weight of 60 g/m2. These surface-treated steel sheets have sufficient corrosion resistance with insufficient electrodeposition painting, which is applied after assembly of automobile bodies, and therefore contribute to prolonged lives of automobiles.
However, the above-mentioned organic composite coated steel sheet has a chromate layer, and if chromium is emitted, it has a great impact on the environment. Therefore, extremely strict wastewater treatment is necessary when the steel sheet is used, which increases the cost.
On the other hand, the heavy coated zinc hot-dip galvanized steel sheet in which chromium is not used has several quality problems, namely, it has poor press formability, and it tends to develop craters during electrodeposition painting and has poor electrodeposition paintability. Furthermore, the heavy coated electrogalvanized steel sheet has poor press formability similar to the thick-coated zinc hot-dip galvanized steel sheet and is too expensive in Japan, where the electricity costs are high. In short, when a steel sheet has a chromate layer which greatly contributes to perforative corrosion resistance, the base plating layer can be thin but it requires measures for environmental protection. On the other hand, when a steel sheet has no chromate layer, the plating layer needs to be thick, which makes the steel sheet inferior in press formability and electrodeposition paintability.
In this connection, a technology to improve perforative corrosion resistance and paint adhesion without increasing the thickness of the plating layer is required. Japanese Unexamined Patent Application Publication No. 52-80239 and Japanese Unexamined Patent Application Publication No. 63-219587 disclose such technologies.
Japanese Unexamined Patent Application Publication No. 52-80239 describes a technology in which a steel sheet or galvanized steel sheet with a coating weight of 10 g/m2 or less is subjected to an iron-based or zinc-based phosphate treatment and is then subjected to a sealing treatment with a silane coupling agent for the purpose of improving paint adhesion. Japanese Unexamined Patent Application Publication No. 63-219587 describes a technology in which a sealing treatment with a silane coupling agent is conducted after a zinc phosphate treatment.
However, no consideration for use as automobile bodies, especially for the recent stringent requirement for corrosion resistance, is given to the coated steel sheets described in the above-mentioned patent application publications. Namely, because the coating weight of zinc is low, sacrificial corrosion prevention by zinc toward iron cannot be exerted for a long time even when the sealing treatment with a silane coupling agent is conducted. Therefore these steel sheets develop corrosion of iron at an early stage, and have poor perforative corrosion resistance.
A silane coupling agent has a functional group which reacts with an inorganic substance (a methoxy group, an ethoxy group, a cellosolve group) and a functional group which reacts with an organic material (a vinyl group, an epoxy group, an amino group, a mercapto group) in a molecule and contributes to adhesion between a metal and an organic paint coating. Therefore, when a steel sheet treated with a silane coupling agent is directly coated with paint, paint adhesion and corrosion resistance after painting are good. However, the silane coupling agent has a shortcoming in that it is dissolved in the chemical conversion treatment because of poor alkali resistance.
In automobile manufacturers, a material is generally subjected to a blanking treatment, then to press forming, and to the chemical conversion treatment. When a large amount of silane coupling agent is dissolved during the chemical conversion treatment, it is natural that sufficient paint adhesion cannot be ensured. In addition, there is a disadvantage in that perforative corrosion resistance cannot be ensured in parts where electrodeposition painting is not sufficiently applied after assembly of automobile bodies.
Incidentally, Japanese Unexamined Patent Application Publication No. 52-80239 discloses evaluation results for a steel sheet which is coated with paint without the chemical conversion treatment after the sealing treatment with the silane coupling agent. Japanese Unexamined Patent Application Publication No. 63-219587 also describes applying paint without the chemical conversion treatment for automobiles.
In particular, the technology described in Japanese Unexamined Patent Application Publication No. 63-219587 involves replacing a phosphate treatment in the automobile manufacturing line with a special phosphate treatment and conducting a sealing treatment with a silane coupling agent in the subsequent treatment step. Therefore, it does not provide a material intended to be subject to blanking, press forming, a chemical conversion treatment, and subsequent various treatments as a material normally used for automobiles. Furthermore, this patent application publication shows corrosion resistance after painting but does not evaluate corrosion resistance in parts without sufficient electrodeposition painting, i.e. perforative corrosion resistance with no paint.
Furthermore, Japanese Unexamined Patent Application Publication No. 59-219478 discloses a treatment agent containing an organoalkoxysilane compound as an aqueous after-treatment agent for metal surfaces subjected to a chemical conversion treatment. Nevertheless, it is difficult for this technology to provide a material intended to be subjected to steps in automobile manufacturers including blanking, press forming, a chemical conversion treatment, and various subsequent treatments as a material normally used for automobiles.
Moreover, Japanese Unexamined Patent Application Publication No. 63-102929 discloses an organic coated steel sheet in which a coating of a ladder-type silicone resin is formed on a chemical-conversion-treated coating. The ladder-type silicone resin as used here is a silicone resin in which siloxane bonds (xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94) form a network structure as below. 
Therefore, the ladder-type silicone resin has superior barrier properties and is effective for improvement of perforative corrosion resistance. However, the resin cannot follow the transformation of the material in forming and, on the contrary, restrains the transformation of the material, which makes the material prone to crack. Therefore, the technology has a disadvantage that satisfactory press formability cannot be obtained.
Accordingly, it is an object of the present invention to provide a coated steel sheet having superior perforative corrosion resistance, paint adhesion, electrodeposition paintability, press formability and weldability which is advantageously adopted as an anti-corrosive steel sheet for automobile bodies, especially without using chromium.
The inventors have intensively investigated how to solve the problems in the conventional art and have invented a surface-treated steel sheet with improved perforative corrosion resistance with no paint and improved paint adhesion, electrodeposition paintability, press formability and weldability.
Accordingly, the inventors have invented a coated steel sheet of a zinc plated steel sheet which has a zinc phosphate-based coating containing magnesium on the surface and a silicone resin coating which has a functional group which reacts with an organic substance on the surface of the zinc phosphate-based coating.
In the coated steel sheet, it is preferred that the zinc phosphate-based coating further contain nickel and manganese because pitting resistance after electrodeposition painting is better in that case. Furthermore, the contents of magnesium, nickel, and manganese in the zinc phosphate-based coating were optimized to reveal that it is preferred that the zinc phosphate-based coating contain 0.5 to 10.0 mass % of magnesium, 0.1 to 2.0 mass % of nickel, and 0.5 to 8.0 mass % of manganese and the manganese and nickel content satisfy the following formula (1), to drastically improve perforative resistance after electrodeposition painting.
[Ni]xc3x977.6-10.9xe2x89xa6[Mn]xe2x89xa6[Ni]xc3x9711.4xe2x80x83xe2x80x83(1)
wherein [Mn] represents the mass % of manganese and [Ni] represents the mass % of nickel.
In addition, the inventors have also found that both perforative corrosion resistance and press formability are improved if the contents of magnesium, nickel, and manganese in the above-mentioned zinc phosphate-based coating are regulated in a specific narrower range, i.e., the above-mentioned zinc phosphate-based coating contains 2.0 to 7.0 mass % of magnesium, 0.1 to 1.4 mass % of nickel, and 0.5 to 5.0 mass % of manganese. The inventors have also found that especially press formability of this coated steel sheet are further improved when zinc phosphate is a granular crystal with a longer side below 2.5 xcexcm in the above-mentioned zinc phosphate-based coating.
The inventors have also found especially press formability are further improved when the silicone resin coating further contains an oxidized polyethylene in all the above-mentioned coated steel sheets.
The present invention has been made based on the above-mentioned findings and it has been made clear that the amount of plating can be reduced without using chromium according to the present invention.